In recent years, manufacturers of large-scale steel pressure vessels (tanks) have been increasingly shifting the sites of assembly of tanks for overseas to the sites of use for the purpose of cost reduction. Heretofore, it has been a general practice to conduct cutting, bending and assembly (assembly by welding) of steel components, the SR process (local heat treatment) of some of the components, and the final assembly in the company's own factory, and then transport the entire tank to the site of use.
However, such contents of work have been changing to the following: for local construction considering efficiency, after only cutting and bending of steel components are carried out in the company's own factory, the materials are transported component by component; the tank is assembled at the site of use (assembly by welding); and the tank is subjected to SR treatment not locally but entirely.
As such a change progresses, because of the problem of the welding technique and from the perspective of safety, increasing the time and numbers of the SR treatment is required. Therefore, a material design which considers conducting the SR treatment for about 20 to 30 hours in total in consideration is required. There has been pointed out the problem that when the long-term SR treatment as described above is carried out, carbide in the steel is agglomerated and coarsened, whereby a reduction in the strength of the steel becomes noticeable.
The rolling method which is a combination of controlled rolling and controlled cooling is referred to as TMCP method, and is widely employed as a method which can provide a steel material having a low carbon equivalent and yet having high strength, high toughness, and high weldability (hereinafter referred to as “TMCP steel”. In addition, the application of TMCP steel have been extended from the steel sheets for welded structures mainly including shipbuilding to the steel sheets for pressure vessels such as tanks. Even in the case of constructing pressure vessels and the like using such TMCP steel, the strength of the steel sheet may be greatly reduced when the long-term SR treatment as described above is carried out.
In order to cope with such situation, a means for imparting high strength is employed prior to conducting the SR treatment in general. However, to achieve high strength under severe conditions of the SR treatment, a large amount of alloy elements needs to be contained therein. As a result, HAZ toughness (in particular, low temperature toughness) of the welded structure is disadvantageously lowered.
Examples of the techniques for suppressing a reduction in strength by the SR treatment as much as possible include patent document 1 which suggests “tough hardened steel for pressure vessels” containing basically 0.26 to 0.75% of Cr and 0.45 to 0.60% of Mo. This technique is for suppressing the coarsening of carbide after the SR treatment by the addition of Cr, and for suppressing a reduction in the strength of the steel after the SR treatment. However, since such a steel material has a large amount of Cr contained, the problem of lowered low temperature toughness of the HAZ remains unsolved.
Patent document 2 suggests “high strength tough hardened steel for pressure vessels” containing basically 0.10 to 1.00% of Cr and 0.45 to 0.60% of Mo. This technique suppresses the reaction of Fe3C with coarse M23C6 by the long-term SR treatment by the addition of Cr. In this technique, the addition of Cr in a relatively wide range is anticipated, but actually only those containing Cr in an amount of 0.29% or higher are shown. It is therefore expected that the low temperature toughness of the HAZ is lowered.    Patent document 1: JP, S57-116756, A    Patent document 2: JP, S57-120652, A